The present invention generally relates to information storage apparatus in a computer system, and particularly to a storage system using detachable disk-like recording media and which can emulate storage apparatus using tape-like recording media.
The storage media for use in a computer system or the like have been developed to have a higher density and capacity year after year. For example, the storage capacity of the 3.5-inch floppy disk used so far has been increased as 360 KB, 720 KB, 1440 KB, and 2880 KB. In addition, the DVD-RAM (Digital Versatile Disc Random Access Memory) that began to be marketed from 1998 has initially a capacity of 2.6 GB, and thereafter its capacity is planned to increase to 4.7 GB and 15 GB. The merit of increasing the capacity of the storage media is to make it possible to increase the recording capacity per installation area of the storage apparatus. This greatly affects the maintenance and management cost of the digital data held by companies.
On the other hand, in the field of large-scale computers, magnetic tapes have been used as recording media for a long time. The magnetic tape has also been improved in its recording density and storage capacity year after year. Therefore, the past assets were stored in low-density magnetic tapes of, for example, 200 MB per cartridge of tape. If the past assets stored in the low-density recording tapes can be transferred to the present high-density magnetic tapes, it is possible to greatly increase the recording density per installation area. Use of, for example, a tape of 50 GB per cartridge will enable data of 250 cartridges of 200-MB magnetic tape to be all transferred to a cartridge of that magnetic tape. If it is simply assumed that the size of a cartridge of high-density magnetic tape is equivalent to that of low-density magnetic tape, the recording capacity per installation area can be increased 250 times. Here, let us consider that information of T1 to T250 (T1, T2 . . . T250 respectively represent a cartridge of conventional magnetic tape) are stored as tape units in a cartridge of the present high-density tape A. In this case, the recording capacity per installation area can be increased as described above. However, if data of T10 and T240 stored on the cartridge of tape A are ordered to read at substantially the same time, the magnetic tape is fast forwarded or rewound, so that the performance of read (access time) of the storage apparatus is greatly reduced. In other words, a cartridge of tape fundamentally cannot respond to the order to read a plurality of times at substantially the same time because of the long latency of the tape-like recording media.
A technique of xe2x80x9cvirtual tapexe2x80x9d as disclosed in, for example, xe2x80x9cVirtual Tape: What It Is And Its Performance Considerationsxe2x80x9d (Kuntal G.Rawal, Steven A. Johnson, CMG97) was contrived to solve this problem and improve the recording capacity per installation area of the storage apparatus. This example describes a method in which a magnetic disk unit is used as a buffer for a plurality of requests to read. Specifically, in the previously given example, when a host computer orders a tape for T10 to be mounted, a virtual tape apparatus mounts magnetic tape A that includes data of T10. Then, a tape label is read from the mounted magnetic tape A, and data position of T10 is obtained. The virtual tape apparatus controls the magnetic tape A to be fast forwarded (or rewound) on the basis of this data position, and the data of T10 to be written in the magnetic disk unit. The host computer orders the data of T10 to be read from the magnetic disk unit in which the data of T10 was written. Then, when a request to read T240 of the same magnetic tape A occurs, the data of T240 is written in the magnetic disk unit. Because the access time of the magnetic disk unit is as extraordinarily fast as 10 ms or below as compared with the magnetic tape, the T10 and T240 can be read in parallel, or at the same time with an appropriate speed for operation even though requests to read the data of T10 and T240 occur at substantially the same time.
In the conventional virtual tape apparatus, the magnetic disk unit is required for the case where data of low-capacity magnetic tapes stored at different positions on a large-capacity magnetic tape are simultaneously read from that large-capacity tape. This results in the increase of the cost of the storage system. In addition, before desired data is read from the magnetic tape, the data is required to be transferred from the magnetic tape to the magnetic disk unit where it is written. Thus, it takes a long time to reach the desired data (the access time is long).
Accordingly, it is an object of the invention to reduce the system cost by removing the necessity of the magnetic disk unit in the conventional virtual tape apparatus.
In addition, it is another object of the invention to reduce the access time of the virtual tape apparatus.
In order to achieve the above objects, according to the invention, there is provided a virtual tape storage system including a control unit connected to a host computer, and a storage unit that is controlled by this control unit to write on or read from removable disk-like recording media in which data positions can be randomly accessed. The control unit has a magnetic tape interface that emulates an interface between the magnetic tape apparatus and the host computer, request translation means for interpreting the command received from the host computer through the magnetic tape interface and generating a command for controlling the storage unit, and controlling means for controlling the storage unit by use of the command generated by the request translation means.
In the preferred embodiments of the invention, a changer is used as the storage unit. The changer has a plurality of cells for holding the removable disk-like recording media, drives for loading the removable disk-like recording media and accessing data on the media according to the order from the control unit, and carrying means for transporting the removable disk-like recording media between the storage housings and the drives. The removable disk-like recording media each have a plurality of recording regions corresponding to the recording media recognized by the host computer and an index region for holding management information that makes identification information of recording media specified by the host computer be associated with the recording regions. The control unit also has a direct interface that receives from the host computer a command for directly accessing the removable disk-like recording media, so that the command from the host computer can be received by either one of the magnetic tape interface and the direct interface in accordance with the selection order from the host computer.